Many wireless / wired routers have a setup option to isolate the wireless clients from the wired LAN. Check your router's documentation.
I found this in the documentation, and I think this is the function you are describing:
WLAN IP Alias
IP alias allows you to partition a physical network into different logical networks over the same Ethernet interface. The ZyWALL supports three logical WLAN interfaces via a its single physical Ethernet with the ZyWALL itself as the gateway for each WLAN network.
What I do not understand is how this is different than the DMZ function which is used to partion a server connected through the Zywall to the internet. Isn't this essentially the same function?